3213 Midterm

Question 1

What is Attenuation(unit)
What is the formula

Answer 1

dB is a measure of attenuation
Attenuation = 10 x log10 (P2/ P1) [dB]
dB measurements in several points are additive/subtractive

Question 2

formula for dBm, dBw

Answer 2

dBw = 10 x log10 (P/1W) unit of Watt
dBm = 10 x log10 (P/1mW) unit of milliWatt
they are units of power

Question 3

What is Delay distortion
Why

Answer 3

Change in signal shape/form
Each signal comp has own propagation speed through medium, therefor different delays

Question 4

What is noise

Answer 4

Unwanted signals that get generated elsewhere, cannot be predicted

Question 5

Signal to noise Ratio (SNR) formula + units
And
(SNRdB or S/NdB )

Answer 5

Ratio of signal power to noise power
avg signal power / avg noise power (watts)

for SNR (decibels)
SNR(db) = 10log10*(SNR)

Question 6

Analog vs Dig transmission (signal reproduction)

Answer 6

Analog: all continuous details must be reproduced accurately
Digital: Only discrete levels need to be reproduced

Question 7

What is a lowpass channel
- range
- dedicated users

Answer 7

[0 - f1) // begins at f=0
dedicated to two devices
devices alternate in transmission

Question 8

What is a bandpass channel?

Answer 8

[f1 - f2) // Does not begin at f=0
medium shared among multiple users
each pair of users get a portion of total bandwidth

Question 9

digital trans pros/cons

Answer 9

pros: does not need to completely recover OG signal shape
thrfor long distance achievable
cons: digital signal theoretically needs f= [0-infinity)
Needs lowpass channel, upper lim can be relax if only using n harmonics

Question 10

Data level vs Signal level

Answer 10

Data levels: bits used to represent data
Signal level: levels allowed in a signal

Question 11

What is Unipolar

Answer 11

Line coding scheme
uses only 1 non-zero voltage lvl, and a 0 lvl
issues: DC comp present, lack of sync for long series

Question 12

What is polar line coding

Answer 12

Line coding scheme
Uses two non-zero voltage levels to represent two data levels
1 pos, 1 neg
DC problem alleviated

Question 13

Types of polar line coding? (4)

Answer 13

Nonreturn to Zero // NRZ (NRZ-level, NRZ-inversion)

Return to Zero // RZ
Manchester (Of type RZ)
Differential Manchester (Of type RZ)

Question 14

What is Polar: NRZ

Answer 14

Nonreturn to zero (name)
NRZ-level: signal lvl represents bit (ex: 0=pos, 1=neg)

NRZ-inversion: inversion of voltage lvl represents bit 1, no voltage change represents bit 0
- long seq of 0 is a problem still, 1 is fine since a change of voltage occurs

Question 15

What is Polar: RZ

Answer 15

Return to zero
0=neg volt, 1 = pos volt
Signal MUST return to zero midway each bit interval
Pros: perfect sync | cons: 2 signal changes to encode each bit, pulse rate = 2x rate of NRZ // More bandwidth req

Question 16

What is Polar: Manchester

Answer 16

Inversion at midpoint of bit interval is for sync and representation
Not exactly return to zero, it is a full inversion of polarity
0=pos to neg trans | 1 = neg to pos trans
perfect sync, but wastes bandwidth for long runs

Question 17

What is Polar: Differential Manchester

Answer 17

Inversion of midpoint of bit used for sync
similar to manchester, BUT 0 = transition, 1 = no transition

Question 18

What is Bipolar

Answer 18

Also known as Alternate Mark inversion (AMI)
Uses two non zero, and a zero voltage (3 levels compared to 2)
0= zero lvl, 1/-1 = alternative pos and neg lvl
If 1 was positive, the next 1 bit will alternate to negative
0 is always 0, therefor long run of 0 is an issue

Question 19

Bipolar solution to long run of 0’s?

Answer 19

Scrambling: Sequences that would result in a long run sync issue are replaced by new sequence
8-zero substitution : 000VB0VB (replaces 0000 0000 sequence)
V = violate alternative 1 pattern | B = don’t violate

Question 20

What is Multilevel?

Answer 20

Data patterns of size 2 bits encoded as one signal element
Requires 4 signal levels for each bit pair combo
data sent 2x faster, half the bandwidth